Method for treating semiconductor slices with gases

ABSTRACT

PROCESS FOR DEPOSITION OF EPITAXIAL AND DIELECTRIC FILMS IS DISCLOSED. MOLYBDENUM DISCS FOR HOLDING SEMICONDUCTOR SILCES ARE MOUNTED FOR ROTATIONAL MOVEMENT IN A NEARVERTICAL PLANE AND FOR TRANSLATION ALONG A LOCUS OF POINTS SUBSTANTIALLY EQUIDISTANT FROM A HEAT SOURCE. THE DISCS IN ONE EMBODIMENT ARE RESTED AGAINST THE INCLINED SIDES OF A ROTATING FRUSTOCONICAL HOUSING WITH THEIR EDGES IN CONTACT WITH A HORIZONTAL TRACK. A DEPOSITION CHAMBER ENVELOPING THE APPARATUS PROVIDES DOWNWARD GAS FLOW. OUTSIDE RF COILS ENCIRCLE THE CHAMBER.

July 20, 1971 o. R. OSWALD METHOD FOR TREATING SEMICONDUCTOR SLICES WITH GASES Filed July 12, 1968 3 Sheets-Sheet 1 FIG.

WLD

ATTORNEY July 20, 1971 D. R. OSWALD 3,594,227

' METHOD FOR TREATING SEMICONDUCTOR SLICES WITH GASES Filed July 12, 1968 3 Sheets-Sheet 2 FIG. 2

July 20, 1971 osw 3,594,227

METHOD FOR TREATING SEMICONDUCTOR SLICES WITH GASES 'Filed July 12, 1968 3 Sheets-Sheet 3 United States Patent O 3,594,227 METHOD FOR TREATING SEMICONDUCTOR SLICES WITH GASES Donald R. Oswald, Schnecksville, Pa., assignor to Bell Telephone Laboratories, Incorporated, Murray Hill,

Filed July 12, 1968, Ser. No. 744,415 Int. Cl. C23c 11/06, 11/00 US. Cl. 117-229 7 Claims ABSTRACT OF THE DISCLOSURE Process for deposition of epitaxial and dielectric films is disclosed. Molybdenum discs for holding semiconductor slices are mounted for rotational movement in a nearvertical plane and for translation along a locus of points substantially equidistant from a heat source. The discs in one embodiment are rested against the inclined sides Of a rotating frustoconical housing with their edges in contact with a horizontal track. A deposition chamber enveloping the apparatus provides downward gas flow. Outside RF coils encircle the chamber.

This invention relates to the processing of semiconductor slices and specifically concerns a method and apparatus for pyrolytic deposition of epitaxial and dielectric films on semiconductor slices.

BACKGROUND OF THE INVENTION In the processing of semiconductor slices for device applications, there are numerous occasions requiring the growth or deposit of a thin oxide or epitaxial film on the slice. In one procedure, for example, semiconductor slices for planar epitaxial devices are coated with an etchable dielectric film such as silicon dioxide to serve as the vehicle for later photolithographic masking. In producing such a film on silicon, it is conventional to thermally oxidize the slices at temperatures of the order of 1100 C. in a water vapor-saturated ambient, using a resistanceheated quartz tube furnace.

A dielectric film can also be realized by vapor deposition of the products of a gaseous reaction at the slice surface. For example, silicon is coated with Si by heating it to about 865 C. in the presence of 0 H and SiCl In general, the process of vapor deposition is faster than thermal oxidation and can be accomplished at a lower temperature; and the dielectric does not have to be a compound formed with the silicon substrate.

Several existing deposition processes with their associated apparatus generally lend themselves to dielectric film deposition on a multislice basis. The familiar quartz tube furnace and epitaxial deposition chamber are two cases in point. As with all semiconductor batch production processes, however, it is necessary to achieve a uniform product. In film deposition, the desired uniformity relates to the film thickness where small variations of even a very few percent often are intolerable.

The problem of film thickness uniformity has several facets. For example, all deposition processes involve a relative movement of the vapor with respect to the slices, coupled with application of the requisite heat. If, during deposition, slices experience gas flow rates or temperatures which dilfer from slice to slice or as to a given slice, the desired thickness uniformity is not achieved. Efiorts to promote even gas flow and eliminate temperature gradients have included shaping the gas stream with bafiles or fans; and causing the slices to move with a planetary rotation within a surrounding RF heating coil. The latter scheme, described in patent application of R. H. Winings, Ser. No. 522,506, filed I an. 24, 1966, now Pat. No. 3,424,- 628, achieves a useful degree of film thickness uniformity,

Patented July 20, 1971 SUMMARY OF THE INVENTION Semiconductor slice holders in the shape of discs are mounted for rotational movement in a near-vertical plane along a locus of points substantially equidistant from a heat source capable of elevating the slice surface temperature to about 1100 C. and in the presence of a continuous flow of depositing gas which impinges upon the discs at a substantial oblique angle.

In one specific embodiment of this invention, semiconductor circular slice holders are mounted for rotation upon a vertical rotating frustoconical housing. As the housing rotates, each slice holder frictionally engages a stationary track and hence rotates about its axis in the fashion of a wheel. The resulting motion of semiconductor slices affixed to the holder has varying components along three mutually orthogonal axes. Then, a depositing gas is directed onto the interior dome surrounding the apparatus, creating the desired gas turbulence and dispersion. The gas flows down and past the inclined moving slice holders. The result is a highly randomized motion of slices in a moving ambient, and the effect is a very uniform thin film deposition.

The inventive practice is achieved as well in a linear movement of rotating slice holders along the length of a quartz tube furnace. The gas flow is directed upon the slices from suitable inlets along the furnace length and at the desired oblique angle.

The invention, its further objects, features and advantages will be better appreciated from a reading of the descriptions to follow of two exemplary methods for the practice thereof.

THE DRAWING FIG. 1 is a frontal perspective view in partial cutaway showing a first item of apparatus designed for practice of the invention;

FIG. 2 is a fragmented sectional frontal view of the apparatus of FIG. 1; and

FIG. 3 is a schematic frontal perspective view of a second item of apparatus designed to practice the invention.

DETAILED DESCRIPTION OF THE PRACTICE OF THE INVENTION FIGS. 1 and 2 illustrate the practice of the invention through the use of a rotating carousel of slice-holding wheels within a controlled gas ambient at elevated temperatures. Within a quartz bell jar 2 having a hemispheric dome 4, there is mounted a central tubular shaft 6. Below the level of dome '4 and disposed concentrically about shaft 6 is a Susceptor assembly 8, so called because it is susceptible to heating by application of radio frequency energy. Susceptor assembly 8 consists of a truncated polysided pyramid r10 advantageously made from molybdenum or silicon-carbide-coated graphite. Pyramid 10 includes a number of symmetrically spaced recessed flat faces 12, all inclined at an angle, designated by the numeral 46, to the vertical axis of shaft 6. Angle 46 is in the range of 5l0 degrees, for example; a range which enhances gas flow patterns, minimizes particle contamina- 3 tion and is consistent with the mechanics of the present embodiment.

Within each recessed face 12 is mounted a disc 14 advantageously made of molybdenum. The rim 16 of each disc 14 rests in frictional contact with a circular stationary track 18. Each disc 14 has a circular recess 20 which receives a semiconductor slice 21. Recess 20 can be produced by etching, for example. Extending from the back of each disc 14 is a central axle 22 that rides in a vertical groove 23 in the susceptor faces 12. A quartz disc 24 is placed between the back side of each disc 14 and the adjacent face 12, to reduce rotational friction of the disc 14 and the face 12, and also to electrically insulate the two from each other to avoid undesired arcing or conduction of RF current when RF heating coils are used.

The hollow molybdenum pyramid 10 is covered by a top plate such as 26 made of quartz. The susceptor assembly is supported on a base 28 which is afiixed to a flange 30 engaged with hub 32. Shaft 6 engages to the interior passage of hub 32, and the entire assembly is affixed to a drive shaft 34 connected to a motor (not shown) exterior to the bell jar 2 through a seal bushing 36.

Also exterior to bell jar 2 at the level of discs 14 is a circular radio frequency coil 38, successive layers of which are outwardly offset to form a truncated column whose apex angle is substantially the same as the angle of incline 46 of the pyramid faces 12. In this fashion the distance between any point on semiconductor slice 21 and the closest part of coil 38 is a constant, since coil 38 is concentrically disposed about the pyramid 10.

A gas entry port 40 is provided at a suitable point along hollow shaft 34 for introduction of deposition gases up through the shaft 34 and shaft 6. Gas enters the interior of jar 2 via the gas inlet 42 from whence the gas flows to the hemispheric dome. At the dome, the gas is uniformly dispersed in an umbrella-like pattern and flows at an acute angle with respect to the faces of slices 21, as indicated by the flow-path arrows 48. The motion in three dimensions of the slices 21 through the gas flow also minimizes the effect of occasional gas composition nonuniformities which occur by imperfect gas mixing. Optimally, the angle which gas flow 48 makes with the surface of slice 21 is in the range of from to 15.

The underlying inventive concept of concurrently rotating and translating semiconductor slices along a locus of points substantially equidistant from a nearby heat source and in the presence of a depositing gas flow whose angle of impingement upon the slices is highly acute, is practiced with the apparatus in FIGS. 1 and 2 by mounting the slices, exhausting the bell jar of air, purging the jar with an inert gas such as helium, causing the susceptor assembly to rotate, energizing the RF coil to bring the assembly to a steady temperature and introducing depositing gases at a predetermined rate. Examples of deposition experiments pursuant to the inventive deposition process as practiced in the apparatus depicted in FIGS. 1 and 2, together with the experimental results are discussed below.

EXAMPLE 1 A number of silicon slices approximately 1 /4 inches in diameter and 6 mils thick which were heavily doped with an antimony type impurity were polished, cleaned and placed in the disc recesses 20 of the susceptor assembly. Rotation of the susceptor assembly at a speed of about 2 r.p.m. was commenced. Bell jar 2 was then purged with N and then H and coil 38 energized with input sufficient to raise the temperature of pyramid to a steady 865 C. after about 10 minutes. Thereafter, a gaseous environment of .O34%f SiH and 2. 57% NH with H carrier was established by introduction of these gases, premixed, into the bell jar 2 through inlet 42 at a rate of about 10 litres per minute. Treatment continued for about minutes until the growth of a layer of .Si N on the surface of the slices to a calculated depth of about 4 0.3 micron. After the slices were cooled and removed, each was examined with a spectrophotometer to determine the degree of uniformity of thickness of the Si N layer. It was found that the difference between the thickest and thinnest portions of the Si N layer deposited over the surface of a given slice was less than .01 micron.

EXAMPLE 2 Slices of silicon 1% inches in diameter and 7 mils thick and having no doping were prepared as in Example 1 and placed in the disc recesses 20. The susceptor assembly was rotated at a speed of about 2 r.p.m. Bell jar 2 was then purged with N and the slice temperature was raised to a steady 900 C. Thereafter, a depositing gas consisting of about 3% CO and .5% AlCl3 with H carrier gas was introduced through inlet 42 at a rate of about 11 litres per minute. After about 1 hour of treatment, the grown layer of A1 0 on the slice surfaces reached a calculated thickness of 2000 A. At this point, the slices were cooled and removed, and each was examined with a spectrophotometer. The difference between the thickest and thinnest portions of the alumina layer was about 50 A.

The proportions used of each gas constituent may, of course, be varied to realize a specific deposition rate or film property. The thickness difference as between individual slices in a given batch produced pursuant to Examples 1 or 2 can be expected to be within 5%. It has been found to be important to closely control the machining and etching of the molybdenum discs to ensure size uniformity among discs; otherwise, disc thickness variations give rise to differing heat transfer rates between disc and slice, which affect the critically temperaturesensitive deposition rate and thus in turn the film thickness uniformity.

The close control in a mass production facility of alumina film thickness to :25 A. uniformly across a slice is a requisite in the making of isolated gate field effect transistors, for example. Thus the inventive process as illustrated through Example 2, supra, can here find Valu able application. The same principles can be applied to perhaps still greater advantage in the further item of apparatus which practices the invention and which now will be described.

FIG. 3 shows an apparatus consisting of an elongated quartz tube furnace 50 in which is mounted a track 52 made of a material that can be heated by RF energy, for example graphite or molybdenum. Track 52 has a face 54 which is offset from the vertical by an angle in the range of from 5 to 10. This angle corresponds to the angle designated 46 mentioned with respect to the FIG. 2 apparatus. Track 52 includes a horizontal runner 56.

Quartz tube line 58 feeds a plurality of gas inlets 60 through the roof of furnace 50, these being spaced at intervals along the hot zone region of the furnace. With one or more gas exhaust outlets approximately diametrically opposite these inlets the preferred gas flow can be made to be across the face 54 at an acute angle as was accomplished in the FIG. 2 apparatus. An RF coil 59 surrounds furnace 50.

At each end of furnace 50 is located a gate 62 which acts as a gas lock to allow entry and exiting of items into the furnace while maintaining a controlled gas condition within. Each gate 62 consists essentially of a housing 64 sealably joined to a diaphragm 6-5 and communicating wit26the furnace interior, and several sliding doors such as Track 52 is mounted within furnace 50 by a support at each end of the furnace. The diaphragm 65 with concentric circular convolutions serves this purpose and seals the ends of the furnace tube 50. The diaphragm can be made from thin stainless steel sheet to be sufficiently flexible in the direction of the furnacetube longitudinal axis so oscillator motion can be transmitted to the track 52 by a vibrator 78 as will be explained below.

Track 52 extends through and beyond the diaphragms 65 in portions 68 exterior to the furnace. Track extensions 68 include an inclined face 70' and a runner 72.

The purpose of tracks 52 and extensions 68, together with their inclined faces 54 and 70, is to facilitate the rolling movement of slice holders 74 and slices 76 into and through the furnace at a slight incline to the vertical so that diffusion gases flowing down through the furnace impinge on slices 76 at a sharp acute angle. In the present embodiment, the scheme for causing the slice holders to roll-and thus the slices to rotate as well as translate involves imparting a patterned vibration to the track 52 via a vibration generator and applicator shown generally as elements 78 and 80 respectively. Movement of slice holders 74 through the gates 62 is realized, for example, by a moving belt system 82 in cooperation with the timed opening of the doors 66. Vibration generators of the type called for would include Syntron feeders which are known in the materials-handling art. The movement of slice holders by vibration has the further advantage of introducing further random motion, albeit of a relatively low amplitude, to the slices, thus further enhancing film thickness uniformity.

The double rotation of semiconductor slices through a gaseous flow in accordance with the teachings of the present invention results in film thickness uniformities superior to those heretofore obtainable by an appreciable factor. An additional advantage derived from practice of the inventive process is that with slices in a near vertical position and the deposition gases flowing over them in an umbrella or curtain pattern in a downward direction, less gas turbulence exists at the slice surface and therefore less likelihood exists of foreign particles settling on the surfaces.

Moreover both dielectric films including SiO Si N A1 and aluminum silicate, and also epitaxial films of silicon a germanium can be grown pursuant to the inventive teachings to a high degree of film thickness uniformity. The invention also permits epitaxial film growth to precede dielectric deposition in one continuous process.

While achieving greater film thickness uniformity, the inventive process compares favorably with results obtained with existing practices in other respects such as haze, stacking faults and etch pits. Resistivities as high as 40 to 50 ohm-centimeters have been obtained with slices whose backs were thermally preoxidized to avoid doping by vapor-phase dopants evolved from the slice backs.

It is to be understood that the embodiments described herein are merely illustrative of the principles of the invention. Various modifications may be made thereto by persons skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. Process for deposition of films on semiconductor slices comprising the steps of:

rotating the slices in a pyrolytic deposition chamber with the slice surfaces at an acute angle of at least 5 with respect to the vertical,

translating the rotating slices along a prescribed path in the chamber, and

applying a flow of film-depositing gas across the slice surfaces in a substantially vertical and downward direction.

2. Process pursuant to claim 1 wherein said prescribed path describes a section of a pyramid surface.

3. Process pursuant to claim 1 wherein said prescribed path is linear.

4. Process pursuant to claim 1 wherein the acute angle gvhich 1126 slices make with respect to the vertical is from 5. Process pursuant to claim 1 wherein the depositing gas is of a composition selected for growing of a dielectric film upon the slice.

6. Process pursuant to claim 1 wherein the depositing gas is of a composition selected for growing of an epitaxial film upon the slice.

7. Process for deposition of thin films on semiconductor slices comprising the steps of:

mounting each slice upon the face of a circular molybdenum disc,

rotating the discs in a pyrolytic deposition chamber in a plane inclined to the vertical by at least 5, translating the rotating discs along a prescribed path in the chamber,

applying RF induction heating to each slice from a source all points of which are always at a substantially uniform distance from the moving slices, the heat being suificient for deposition of films from a gas, and

applying a stream of film-depositing gas downwardly upon the moving substrate, its direction of flow impinging upon the slice surfaces at an angle of from 5 to 10.

References Cited UNITED STATES PATENTS 3,208,888 9/1965 Ziegler et a1 148--174X 3,365,336 1/1968 Folkmann et a1 l48-175 3,424,628 1/ 1969 Winings 117106 3,424,629 l/1969 Ernst et a1 117--106 3,464,846 9/1969 Mattson 1l7-107.1

ALFRED L. LEAVITT, Primary Examiner E. G. WHITBY, Assistant Examiner US. Cl. X.R.

117Digest 12, 93.2, 106, 107.1, 107.2, 201; 1 48175; 29-583 

